1. Field of the Invention
This invention relates to a plate for a spin drum in a centrifuge. Each plate has spacers which are fixed on the plate and maintain an intermediate space between two or more stacked plates. The invention also includes a process for the manufacture of these plates with spacers.
Generally, centrifuges are formed by a large number of plates stacked one on top of the other in the shape of a truncated cone. These plates are separated from each other by spacers that form intermediate spaces between the plates. In these intermediate spaces, the liquid to be treated is divided in a correspondingly large number of thin layers. The liquid is separated because these liquid components have different densities.
Unfortunately, mounting spacer strips on plates can be very costly. To minimize costs, spot welding has been found to be an economically justifiable process for mounting the spacer strips, and is employed in most cases. However, the spot welding process has one drawback in that fine gaps remain between the surface of the plate and the spacer strips. These gaps cannot be cleaned sufficiently even with intensive circulatory cleaning as is usually applied. Thus, such gaps cause the formation of an undesirable culture medium in which centers of bacteria may develop to contaminate the drum. In products that are expected to meet high hygienic requirements, this bacteria may lead to undesirable contamination of the separated or clarified product. In turn, this contamination may render the product unusable especially in the field of biotechnology.
2. The Prior Art
So as to avoid such contamination, it is known to make the applied spacer strips liquid-tight against the surface of the plate along their longitudinal and cross sides. As described in German Patent DE 557 376 and French Patent FR 464 116, this is accomplished by liquid-tight soldering around the edges of the spacer strips. However, there are two drawbacks to this finishing step. First, the solder is not resistant to aggressive liquids of the type employed in circulation cleaning. Second, additional soldering of the edges of the spacer strips is very expensive.
German Patent No. DE 40 36 071 shows welding of liquid tight spacer strips to the surface of the plate by of a roll welding machine. In addition, German Patent DE 40 36 071 also shows how to weld the spacer strips liquid-tight along their edges with laser welding equipment. It is possible with both welding methods to finish the gaps liquid-tight with welding seams which are resistant to aggressive liquids employed in chemical circulation cleaning. However, these two methods for welding spacers to plates can have many drawbacks.
For example, producing the plates with these welding methods is highly time-consuming and very costly. Furthermore, since there is a large number of spacer strips required for each plate and a large number of plates, this can greatly increase the cost of a centrifuge. These high manufacturing costs may limit the use of these plates to purification processes in a contamination free environment such as in the field of biotechnology.
In addition, roll welding involves substantial deformation of the plate, which may impair the function of the plate. This problem may be so great so as to render the plate unusable. Moreover, when the plates are stacked on top of each other, it is not always possible to ensure that the spacer strips of the individual plates cover each other. These spacer strips may not match with the adjacent plates due to manufacturing tolerances and inaccuracies when the spacer strips are mounted, and the play or clearance required for plugging to the distributor. These spacer strips may be displaced relative to one another to such an extent that individual spacer strips of a plate are supported only on part of the adjoining spacer strip of the adjacent plate. Under certain circumstances, this result may render individual spacer strips ineffective due to the clamping force acting on the plates as the set of plates is being braced, which may lead to undesirable deformation of the surface of the plate. To avoid this result, spacer strips are designed with a width of from 4 to 7 mm. This width range of the spacer strips, which is actually not required for the separation effect, necessarily reduces the surface area available for separation and also clarification of products.
Finally, displacement of the spacer strips may also lead to the formation of micro-gaps between the strips and the adjacent plate surface. Unfortunately, these micro-gaps cannot be closed in spite of the acting clamping pressure. Such micro-gaps, however, may cause contamination of the product treated because such a fine gap structure cannot be engaged or reached in the usual circulation cleaning of centrifuges. Dismantling of the drum for the purpose of thorough manual cleaning is consequently unavoidable when biotechnological products are separated or clarified.
It is known from German Patent DE 17 02 178 Ul to adjust the spacing required between two adjacent plates by embossing from the plate materials. However, this embossing is not capable of counteracting the high pressure by which the plates are pressed together in the centrifuge drum. The consequence thereof is that the stamped structures are forced or pressed back, so that it is not possible to maintain the predetermined spacing between adjacent plates. In addition, there is the risk that displacement is caused within the set of plates, which may lead to strong imbalance of the centrifuge drum.
Therefore, one object of the present invention is to provide the plate specified above with spacers which can be applied to the plate free of gaps and in a manner that is stable under pressure without additional materials.
In addition, another object of the invention is to provide a centrifuge plate that is simple in design, easy to install, and inexpensive to manufacture.
These objects according to the invention are achieved in that the spacers are material thickenings of the plate itself. Forming the spacers integrally with the plate assures that the spacers are applied to the plate free of gaps. Since the spacers are molded with the plate, the plate consists of a homogeneous material quality. This eliminates the necessity of welding materials.
In addition, forming the plates with the spacers can reduce cracking in the connection seams. Cracking is caused by possible distortions of the plate, which results in a separation of the plate from the spacers at connection seams. Therefore, by forming the spacers to the plates, connection seams are eliminated and cracking is reduced.
Furthermore, the spacer according to the invention adjoins the adjacent plate smoothly and abuts on the surface of the adjacent plate free of gaps after the set of plates has been compressed. As a result of forced material thickening, the intermediate space between the surface of the spacer and the surface of the plate is completely filled and forcing or pressing back of the spacers is prevented. This assures a safe function of the plate without the risk that the spacers may be forced into a position in which the function of the centrifuge drum could be impaired.
According to a preferred embodiment of the invention, the material thickening is shaped on the bottom side of the plate. It is possible to inexpensively manufacture the plate with material thickenings shaped as spacers by roll pressing.
To manufacture the plate inexpensively it should be roll pressed. In addition, with this manufacturing process, the positions of the spacers are distinctly reproducible on each individual plate.
The roll pressing procedure is achieved by pressing the blank across a rolling cone shaped mandrel. The cone forms depressions conforming to the shapes of the spacers of the plate. These depressions are filled with the material of the blank.
This process assures that material thickenings shaped by impression are distinctly reproducible on each individual plate. Since these plates are uniform in design, they stack so that their spacers match with an adjacent plate across their entire surface. There is only minor displacement of adjacent spacers caused by play or clearance required for plugging the plates on the distributor. Therefore, undesirable deformation of the plate within this zone and formation of un-cleanable micro-gaps can be almost completely excluded.
Since the spacers are in an almost uniform position on each plate, it is possible to substantially reduce the width of the spacers and to thereby enlarge the surface area available on each plate for separation or clarification purposes. Moreover, this new procedure removes the additional manufacturing steps such as tacking and liquid-tight welding of the edges of the spacers. Therefore, the manufacturing costs of a plate and thus of a set or pack of plates for a centrifuge drum is significantly reduced.
The side of the depressions directed at the small diameter of the cone-shaped mandrel ends approximately parallel with the center axis of the cone-shaped mandrel. In this way, the pressed plate with the spacers shaped on the bottom side can be easily removed from the cone-shaped mandrel even if spacers with a greater height have to be produced.
In the first stage of the process, the blank is heated prior to roll pressing. However, it may be more advantageous to point or spot heat the roll with a laser beam upstream of the pressing roll. When heat is applied to each blank, it enhances the flow properties of the material on the plate. Since the flow properties are enhanced, this reduces the contact pressure from the cone-shaped mandrel needed to form the spacings. In addition these enhanced flow properties reduce the amount of time that the cone-shaped mandrel needs to form the spacings.
In addition, to facilitate this process, the blank has a thickness that is greater than that required for the final plate. This assures that a pressing bead can develop that is suitable for supplying an adequate amount of material for the spacers. The thickness required for the blank has to be selected prior to roll pressing, and depends on the desired height of the spacers and the wall thickness intended for the finished plate. That way the plate does not have to be reworked.
According to yet another embodiment the process as defined by the invention, a blank which, on the side facing the cone-shaped mandrel, is provided with material accumulations which, during pressing of the plate, are approximately disposed within the zone of the depressions of the cone-shaped mandrel and are pressed into the depressions as material thickenings. The contact pressure of the pressing roller can thus be very favorably selected.
Since the plates are uniformly pre-cast and pre-heated before they are rolled, upon rolling, the spacers are clearly shaped in the same positions in each individual plate produced. Consequently, the surfaces of the spacers of the plates can be shifted or displaced in only a minor way. This minor shifting occurs during clamping of the set of packaged plates on account of the small amount of play required for plugging on the distributor. Such minor displacement, however, will not lead to any formation of micro-gaps that cannot be cleaned, or to any undesirable deformation of the plates. Furthermore, this process dispenses with complicated mounting procedures and liquid-tight welding of the spacer edges extending around a multitude of spacer strips. Both of these processes are very time-consuming and costly, and do not eliminate the risk of inaccurate positioning of the spacer strips.